Biomass as a Nonfossil Fuel Source - American Chemical Society

9 Key Factors in the Hydrolysis of Cellulose Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 24, 2015 | http://pubs.acs.org Publication Date: January 29, 1981 | doi: 10.1021/bk-1981-0144.ch009

JEROME F. SAEMAN Forest Products Laboratory, Forest Service, USDA, P.O. Box 5130, Madison, WI 53705

If a successful cellulose hydrolysis process were available, the food problems of the world would diminish and wood sugar could form the basis of new enterprises providing an important source of chemicals and fuels. This has been known for more than a century. Some hundred to perhaps hundreds of millions of dollars have been spent on research dealing with cellulose hydrolysis. Billions have been invested in commercial plants for the hydrolysis of wood. Success, however, has been limited and future prospects are uncertain. A review of early wood hydrolysis processes was published by Sherrard and Kressman (1). A more recent review was published by Wenzl (2). The coverage here is selective rather than comprehensive. It identifies some key issues that determine the future usefulness of wood hydrolysis processes. Cellulose is a polymeric carbohydrate (C H O ) having the same elemental composition as starch and also yielding only glucose on complete hydrolysis. Cellulose consists of long chains of beta glucosidic residues linked through the 1,4 positions. Starch consists of alpha glucosidic residues linked through the same positions. These linkages are as similar as right and left hands, but differences in overall configuration cause cellulose to have a high crystallinity and hence a low accessibility to enzymes or acid catalysts. Starch by contrast is very readily hydrolyzed, suiting its role as a form of stored food. 6

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This chapter not subject to U.S. copyright. Published 1981 American Chemical Society

In Biomass as a Nonfossil Fuel Source; Klass, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 24, 2015 | http://pubs.acs.org Publication Date: January 29, 1981 | doi: 10.1021/bk-1981-0144.ch009

Cellulose fibers p r o v i d e for t h e s t r u c t u r a l needs of plants. Hemicellulose. an easily h y d r o l y z e d a m o r p h o u s h e t e r o - p o l y m e r y i e l d i n g several different sugars on hydrolysis, is i n t i m a t e l y associated w i t h cellulose. L i g n i n , an a r o m a t i c t h r e e d i m e n s i o n a l p o l y m e r , intersperses t h e f i b e r o u s c o n s t i t u e n t s . M a n y w o o d s c o n s i s t of a little less t h a n half cellulose a n d r o u g h l y a q u a r t e r each of h e m i c e l l u l o s e a n d l i g n i n . Cellulose is of t h e order of a h u n d r e d t i m e s as d i f f i c u l t t o h y d r o l y z e as s t a r c h . L i g n i n has little effect if a n y on t h e rate of a c i d hydrolysis, b u t it g r e a t l y i n h i b i t s e n z y m a t i c hydrolysis. This p r o t e c t s t h e p l a n t against its ready use as a f o o d b y o t h e r o r g a n i s m s . DILUTE A C I D HYDROLYSIS OF CELLULOSE M o s t s i m p l e o l i g o s a c c h a r i d e s are q u a n t i t a t i v e l y h y d r o l y z e d by b o i l i n g d i l u t e a c i d . Cellulose by c o n t r a s t is h y d r o l y z e d very s l o w l y , a n d even o n e x t e n d e d h y d r o l y s i s at h i g h e r t e m p e r a t u r e s , t h e m a x i m u m y i e l d of recoverable sugar is very low. This w a s e x p l a i n e d by Luers (3) w h o s h o w e d t h e cellulose h y d r o l y s i s in d i l u t e acid involves c o n s e c u t i v e first-order reactions of s o m e w h a t similar rates for t h e p r o d u c t i o n a n d t h e d e c o m p o s i t i o n of sugar. T h i s o b s e r v a t i o n f o r m e d t h e basis for t h e d e v e l o p m e n t of t h e Scholler p e r c o l a t i o n process. This process, w h i c h involves r e m o v a l of sugar f r o m t h e d i g e s t e r as it is f o r m e d , resulted in t w i c e t h e y i e l d o b t a i n a b l e by b a t c h hydrolysis. Luers erred, h o w e v e r , in c o n c l u d i n g t h a t c h a n g i n g t h e acid c o n c e n t r a t i o n a n d t e m p e r a t u r e c h a n g e d t h e rate b u t n o t t h e c o u r s e of t h e reaction nor t h e m a x i m u m y i e l d a t t a i n a b l e . S a e m a n (4) s h o w e d t h a t t h e h y d r o l y s i s reaction is a c c e l e r a t e d m o r e t h a n t h e d e c o m p o s i t i o n r e a c t i o n b y b o t h increased t e m p e r a t u r e a n d acid c o n c e n t r a t i o n . Hence, t h e sugar y i e l d increases w i t h b o t h acid c o n c e n t r a t i o n a n d t e m p e r a t u r e . This o b s e r v a t i o n is a p p l i c a b l e t o b o t h t h e p e r c o l a t i o n process a n d t o t h e m u c h s i m p l e r b a t c h process. SINGLE-STAGE DILUTE ACID HYDROLYSIS Figure I s h o w s t h e m a x i m u m y i e l d of " B " a n d t h e residual " A " in c o n s e c u t i v e f i r s t - o r d e r reactions: A X B l4 C for v a r i o u s ratios of k : k - kr. Parameters for cellulose h y d r o l y s i s w e r e d e t e r m i n e d by S a e m a n (4) a n d r e d e t e r m i n e d by Kirby j 5 ) . It m i g h t be n o t e d t h a t d e s p i t e a n a l y t i c a l a n d e x p e r i m e n t a l p r o b l e m s , t h e general m o d e l has been repeatedly c o n f i r m e d . k

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U s i n g Kirby's d a t a a n d f a m i l i a r first order kinetics, t h e r e l a t i o n s h i p s h o w n in Figure II w a s d e r i v e d . T h i s w a s t e s t e d at t e m p e r a t u r e s u p t o 2 8 0 ° C ( c o r r e s p o n d i n g t o 9 1 6 l b s / s q in. e q u i l i b r i u m s t e a m pressure) in 0.25 i n c h d i a m e t e r c o p p e r t u b i n g reactors. In these b a t c h e x p e r i m e n t s , 0.6 g of


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p r e h y d r o l y z e d w o o d w a s reacted w i t h 1.5 m l of d i l u t e s u l f u r i c acid in an a t m o s p h e r e of c a r b o n d i o x i d e . T h e reactors w e r e heated w i t h s u i t a b l e p r e c a u t i o n s in a m o l t e n salt b a t h a n d q u e n c h e d in w a t e r . A s s h o w n i n Figure III, p r e d i c t e d general i m p r o v e m e n t in yield w a s m a i n t a i n e d u p t o 2 6 0 ° C . A t t h i s p o i n t , t h e y i e l d w a s 5 4 % . T h e t i m e t o m a x i m u m y i e l d at 2 6 0 ° C w a s 0.45 m i n u t e s . T h e m o d e l p r e s u m e s i s o t h e r m a l reaction c o n d i t i o n s , b u t w i t h a r e a c t i o n t i m e o f 3 0 s e c o n d s , t h e average reaction t e m p e r a t u r e w a s m u c h b e l o w t h e b a t c h t e m p e r a t u r e , t h u s r e s u l t i n g in l o w e r yields. A n e x a m i n a t i o n o f t h e kinetics s h o w s t h a t if t h e l o w e r l i m i t o f p r a c t i c a l r e t e n t i o n t i m e is r e a c h e d , it is a d v a n t a g e o u s t o l o w e r t h e a c i d i t y a n d raise t h e t e m p e r a t u r e . It is also e v i d e n t t h e p r e t r e a t m e n t s w h i c h accelerate t h e rate o f h y d r o l y s i s a n d increase t h e heat o f a c t i v a t i o n are a d v a n t a g e o u s . M i c r o t e c h n i q u e s are n o w available w h i c h c a n establish yields o n m i l l i g r a m samples h e a t e d in glass capillaries w i t h reaction t i m e s o f a f e w s e c o n d s a n d internal pressure in excess o f a 1 0 0 0 l b s / s q in b u t t h e y have n o t y e t been a p p l i e d t o t h i s p r o b l e m . P u m p e d h y d r o c e l l u l o s e slurries c a n b e heated b y s t e a m i n j e c t i o n a n d q u e n c h e d b y release o f pressure, b u t t h e p r a c t i c a b i l i t y of s u c h a p r o c e s s i n g t e c h n i q u e has n o t been e s t a b l i s h e d . G r e t h l e i n r e c e n t l y c o n f i r m e d S a e m a n ' s m o d e l by r e p o r t i n g a y i e l d o f over 5 0 % sugar f r o m cellulose u s i n g 1 % s u l f u r i c a c i d a n d a c o n t i n u o u s - f l o w reactor w i t h a residence t i m e o f 0.22 m i n u t e s at a t e m p e r a t u r e o f 2 3 7 °C (6). Research seeking h i g h e r y i e l d s b y t h i s a p p r o a c h s h o u l d be f r u i t f u l . DILUTE

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PROCESSING Early a t t e m p t s at d i l u t e a c i d h y d r o l y s i s b y single-stage p r o c e s s i n g w e r e l i m i t e d t o yields o f a b o u t 2 0 % f e r m e n t a b l e s u g a r b e c a u s e o f t h e u n f a v o r a b l e k IELD

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Figure 5. Overall sugar yield obtained by hydrolysis, degree of polymerization, and content of undecomposed carbohydrate as functions of irradiation dose (11)

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Spano reported t h a t cellulose p r e t r e a t e d b y r o l l - m i l l i n g w i t h a n energy i n p u t of 0 . 2 5 k i l o w a t t p e r h o u r p e r p o u n d w a s e n z y m a t i c a l l y h y d r o l y z e d t o t h e e x t e n t o f 4 5 % in 2 4 hours. T h e results o f t h i s w e r e said t o b e e n c o u r a g i n g (13). E N Z Y M A T I C HYDROLYSIS OF CELLULOSE


There is m u c h e x c e l l e n t basic w o r k u n d e r w a y o n t h e e n z y m a t i c hydrolysis o f cellulose. The key f a c t o r is t h e p r e t r e a t m e n t r e q u i r e d b y lignocellulose before it is a c t e d u p o n a t an a c c e p t a b l e rate. In p a r t i a l l y d e l i g n i f i e d p u l p s , t h e rate o f h y d r o l y s i s rises w i t h d e c r e a s i n g l i g n i n c o n t e n t , b u t p u l p i n g t h e s u b s t r a t e is impractical. P r e t r e a t m e n t s m e n t i o n e d previously i n c l u d e irradiation, b a l l - m i l l i n g , a n d rollm i l l i n g . S o m e success in u p g r a d i n g r u m i n a n t f o d d e r has been a c h i e v e d w i t h s t e a m i n g , t r e a t m e n t w i t h d i l u t e alkali, a n d d i g e s t i o n w i t h a q u e o u s sulfur d i o x i d e solutions. Intensive w o r k d i r e c t e d t o t h e d e v e l o p m e n t o f inexpensive solubilization m e t h o d s f o r increasing t h e y i e l d o f sugar f r o m lignocellulose b y acid or e n z y m a t i c hydrolysis is in progress at Purdue University. A recent report s h o w e d t h a t c a d o x e n is e f f e c t i v e b u t c a d m i u m presents u n a c c e p t a b l e hazards (14). A n a p p r o a c h in w h i c h p r e h y d r o l y z e d lignocellulose is t r e a t e d w i t h 7 0 % sulfuric acid f o l l o w e d b y m e t h a n o l has also been r e p o r t e d (14). T h e f l o w c h a r t calls f o r r e c y c l i n g 2.5 parts o f s u l f u r i c a c i d a n d 5.5 parts o f m e t h a n o l for each part o f sugar p r o d u c e d . A n o t h e r a p p r o a c h (15) calls for t h e use o f a solvent c o n s i s t i n g o f s o d i u m tartrate, ferric c h l o r i d e , s o d i u m sulfite, a n d s o d i u m h y d r o x i d e . S u c h t r e a t m e n t s increase t h e accessibility o f e n z y m e s t o cellulose a n d increase t h e rate o f a c i d hydrolysis, t h e k : k ratio, a n d t h e y i e l d o f sugar. T h e cost o f r e c o v e r i n g t h e reagents, h o w e v e r , is a key factor. W h i l e t h e r e is w i d e s p r e a d e n t h u s i a s m f o r t h e e n z y m a t i c hydrolysis o f lignocellulose a n d many pretreatments facilitate the reaction, published process d a t a d o n o t p e r m i t e c o n o m i c assessment. }

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CONCLUSIONS T h e o u t l o o k f o r fuels a n d c h e m i c a l s f r o m b i o m a s s is c l o u d e d because it d e p e n d s o n t h e o u t l o o k f o r energy. T h e m o s t p r o b a b l e o u t l o o k for e n e r g y in t h e n e x t d e c a d e is t h a t it w i l l be e x p e n s i v e b u t available, a n d c o n s u m p t i o n w i l l c o n t i n u e t o increase. W h i l e t h i s s i t u a t i o n m a y n o t c o n s t i t u t e a crisis, it is "crisis p r o n e . " This latter f a c t justifies a careful c o n s i d e r a t i o n of biomass as a source of fuels a n d c h e m i c a l s .


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The price of b i o m a s s is critical in d e t e r m i n i n g its p o t e n t i a l . Because of lack of d e m a n d , c u r r e n t s p o t prices are o f t e n l o w . T h e price of a large q u a n t i t y of w o o d (a t h o u s a n d or m o r e t o n s / d a y ) for t h e life of a p l a n t (decades) is n o t l o w . Katzen in 1 9 7 5 (7) a s s u m e d t h a t a large q u a n t i t y of w o o d w i l l have a m i n i m u m v a l u e of $ 2 4 / t o n (dry basis) set b y its fuel e q u i v a l e n t , a n d t h e a d d i t i o n a l cost of assuring l o n g - t e r m s u p p l y w o u l d raise t h i s t o $ 3 6 / t o n . The


cost of w o o d , t o g e t h e r w i t h labor a n d capital costs, w i l l of course rise as t h e cost of p e t r o l e u m rises. T h e best use of b i o m a s s f o r all purposes requires realistic, d i s c r i m i n a t i n g , a n d selective s e t t i n g of priorities. W o o d c o n t r i b u t e s m o s t t o o u r e n e r g y b u d g e t w h e n it is used for t h e p r o d u c t i o n of s t r u c t u r a l a n d fiber p r o d u c t s , t h u s p r o v i d i n g a l t e r n a t i v e s t o e n e r g y - i n t e n s i v e materials. A t o n of w o o d in t h e materials s y s t e m c o n t r i b u t e s i n d i r e c t l y m a n y t i m e s as m u c h t o o u r n a t i o n a l e n e r g y b u d g e t as a t o n of w o o d in t h e fuel s y s t e m . T h e relative e c o n o m i c i m p o r t a n c e of forest p r o d u c t s w i l l increase as t h e e n e r g y s h o r t a g e w o r s e n s , a n d as forest p r o d u c t s i n d u s t r i e s accelerate t h e i r s u b s t i t u t i o n of l o w - g r a d e w o o d for fossil fuel. A f t e r s a t i s f y i n g t h e needs for an e n e r g y self-sufficient w o o d i n d u s t r y , t h e r e c o u l d still be available s o m e h u n d r e d or h u n d r e d s of m i l l i o n s of t o n s of b i o m a s s in t h e f o r m of w o o d , t o g e t h e r w i t h a similar q u a n t i t y of a g r i c u l t u r a l residues. T h e large-scale c h e m i c a l c o n v e r s i o n of s u c h r a w material w i l l p r o b a b l y i n v o l v e h y d r o l y s i s , b u t no available t e c h n o l o g y is c o n s i d e r e d e c o n o m i c a l l y viable. T h e key t o f u t u r e progress lies in basic s t u d i e s , a n d an e v o l u t i o n a r y a p p r o a c h t o possible large-scale i n t e g r a t e d c h e m i c a l utilization. Cellulose h y d r o l y s i s by m e a n s of e n z y m e s , s t r o n g a c i d , or d i l u t e acid is preferably p r e c e d e d by a p r e h y d r o l y s i s t o separate easily h y d r o l y z e d h e m i c e l l u l o s e c o n s t i t u e n t s . A s an e v o l u t i o n a r y s t e p , p r e h y d r o l y s i s itself m i g h t be a v i a b l e route for a l i m i t e d q u a n t i t y of special p r o d u c t s . Prehydrolysis Prehydrolysis is a s i m p l e a n d w e l l - k n o w n step r e q u i r i n g little f u r t h e r basic studies, b u t t h e r e are solvable t e c h n i c a l p r o b l e m s in o b t a i n i n g t h e p r e h y d r o l y z a t e s in f a v o r a b l e c o n c e n t r a t i o n . Prior t o t h e e s t a b l i s h m e n t of a f u l l y i n t e g r a t e d h y d r o l y s i s o p e r a t i o n , t h e r e m i g h t be o p p o r t u n i t i e s t o p r o d u c e a n d p u t t o use h a r d w o o d or c r o p - r e s i d u e prehydrolyzates. There are n o w in p r o s p e c t boiler p l a n t s w h i c h w i l l b u r n over 1,000 t o n s of w o o d / d a y . Preceding c o m b u s t i o n , w o o d can be p r e h y d r o l y z e d t o y i e l d a s t r e a m rich in pentoses for s u b s e q u e n t c o n v e r s i o n t o xylose, xylose


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derivatives, f u r f u r a l , yeast, or o t h e r feed a n d f e r m e n t a t i o n p r o d u c t s 0 6 ) . T h e residual w o o d , a b o u t t h r e e - f o u r t h s o f t h e i n c o m i n g w e i g h t , c a n t h e n be b u r n e d f o r process s t e a m a n d a d d i t i o n a l s t e a m as a c o p r o d u c t . S u c h an o p e r a t i o n w o u l d be a n e v o l u t i o n a r y step t o w a r d an i n t e g r a t e d hydrolysis plant.


Pretreatment for Enzymatic Hydrolysis W h i l e t h e r e has been m u c h progress in t h e s t u d y o f cellulases, t h e a p p l i c a t i o n s of s u c h t e c h n o l o g y have been l i m i t e d by a lack o f e c o n o m i c a l p r e t r e a t m e n t of t h e lignocellulose. W i t h o u t s u c h p r e t r e a t m e n t , hydrolysis is s l o w a n d i n c o m p l e t e . T h e v a l u e of e n h a n c e d e n z y m a t i c a t t a c k o n lignocellulose is n o t l i m i t e d t o t h e p r o d u c t i o n o f sugar a n d c h e m i c a l s . T h e same p r o c e d u r e s w o u l d be a p p l i c a b l e t o t h e increased d i g e s t i b i l i t y o f coarse fodder by ruminants. W h i l e g o v e r n m e n t - s u p p o r t e d research n o w emphasizes t h e p r o d u c t i o n o f l i q u i d fuels f r o m b i o m a s s , c o m m e r c i a l i z a t i o n m i g h t be reached sooner b y c o o p e r a t i o n w i t h t h o s e interested in cellulose d i g e s t i o n b y r u m i n a n t s . Experience in t h e practical u p g r a d i n g o f coarse f o d d e r w o u l d be d i r e c t l y a p p l i c a b l e t o t h e h y d r o l y s i s of b i o m a s s b y cellulases. The Strong Acid Hydrolysis o f Cellulose The h i g h yield a n d h e n c e h i g h e r p u r i t y o f sugar o b t a i n e d b y s t r o n g acid h y d r o l y s i s o f cellulose makes it a n a t t r a c t i v e process, b u t t h e lack of a recovery s y s t e m f o r s t r o n g acid c o m p l i c a t e s t h e outlook. T h e i m p r o v e m e n t o f m e m b r a n e t e c h n o l o g y w i l l p r o b a b l y p r o c e e d because o f p o t e n t i a l a p p l i c a t i o n s t o m a n y p r o b l e m s . A p p l i c a t i o n t o cellulose hydrolysis a d d s j u s t i f i c a t i o n for i n t e n s i f i e d w o r k in t h e field. The Dilute Acid Hydrolysis of Cellulose T h e r a p i d , h i g h - t e m p e r a t u r e h y d r o l y s i s o f cellulose seems t o g e t less a t t e n t i o n t h a n it deserves. T h e s e q u e n c e i n v o l v e d is s i m p l e ; t h e i n c o m i n g w e t m a t e r i a l need never be d r i e d . T h e c o n s e c u t i v e first-order reactions i n v o l v e d c a n b e s t u d i e d w i t h a d e q u a t e precision in very s i m p l e e q u i p m e n t . W h i l e t h e o u t l o o k f o r t h e process, based o n m e a g e r presently available d a t a , is m a r g i n a l , studies c a n be c o n d u c t e d t o increase t h e ratio o f t h e rate of sugar p r o d u c t i o n t o d e s t r u c t i o n a n d h e n c e t h e y i e l d , t o decrease t h e t e m p e r a t u r e a n d pressures i n v o l v e d , a n d t o increase t h e recovery o f c o p r o d u c t s .


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REFERENCES

1. Sherrard, E. C.; Kressman, F. W. Ind. Eng. Chem. 1945, 37, 5.


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Biomass as a Nonfossil Fuel Source - American Chemical Society

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